Amer Networks SS3GR24i - Manual (Page 41)

SS3GR24i/SS3GR48i Layer3 Stackable Gigabit Switch User’s Guide
Many levels of encapsulation are required for a packet to cross the
Internet.
Packet Headers
TCP
Most data transmissions are much longer that a single packet. The
data must then be divided up among a series of packets. These
packets must be transmitted, received and then reassembled into
the original data. TCP handles these functions.
TCP must know how large a packet the network can process. To do
this, the TCP protocols at each end of a connection state how large
a packet they can handle and the smaller of the two is selected.
The TCP header contains at least 20 octets. The source and
destination TCP port numbers are the most important fields.
These specify the connection between two TCP protocols on two
network devices.
The header also contains a sequence number that is used to ensure
the packets are received in the correct order. The packets are not
numbered, but rather the octets the packets contain are. If there
are 100 octets of data in each packet, the first packet is numbered
0, the second 100, the third 200, etc.
To insure that the data in a packet is received uncorrupted, TCP
adds the binary value of all the octets in the packet and writes the
sum in the checksum field. The receiving TCP recalculates the
checksum and if the numbers are different, the packet is dropped.
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